Historically, turboprop engines have been controlled via two levers in the cockpit. A speed lever adjusts engine power-turbine speed and a power lever controls engine power. The pilot would adjust the speed lever to obtain the desired engine speed setting. Then, the pilot "closes the loop" on torque by watching a gauge and moving the power lever angle to the desired torque level. Engine power-turbine speed increases until the desired speed is reached. As the power lever is advanced further, engine speed remains constant, but engine torque would be further increased.
Propeller shaft speed is controlled by a power-turbine speed governor. A torque motor on the governor allows for the adjustment of the propeller shaft speed. The speed lever sets the governor to a desired speed. As the power lever is adjusted, the governor adjusts the pitch of the blades to hold the engine at the desired speed. The governor regulates propeller blade pitch by controlling the pressure of oil supplied to a propeller speed. By increasing the pressure of oil supplied to the propeller dome, blade pitch is reduced; and by decreasing the pressure of oil supplied to the propeller dome, blade pitch is increased.
Among the problems associated with the governor control loops is propeller overshoot. When the engine is commanded to accelerate, much added energy is required to accelerate the propeller to the commanded speed. Once the power turbine reaches the commanded speed, the governor begins to change blade angle. Due to the proportional regulation action of the power turbine speed governor and the inertia of the propeller, however, the engine overshoots its commanded speed.
Therefore, it is an object of the present invention to prevent propeller overshoot.